Why does the adsorption and concentration of inorganic chemical species proceed at aqueous–solid interfaces? In this review paper, we discuss the use of X-ray chemical state analysis to elucidate the intrinsic adsorption mechanism. Based on the chemical states of the species adsorbed to solids as determined by X-ray chemical state analysis, possible adsorption mechanisms are discussed. The driving forces of adsorption are represented by the Gibbs free energy change (DGchem = DGchem,1 + DGchem,2) resulting from the formation of covalent bonds between metal ions (M) in metal oxides or hydroxides and adsorbed species (X) (M-O-X bond, DGchem,1) and the formation of new phases consisting of M and X (DGchem,2). The concept of DGchem, 2 is proposed based on the experimental results from chemical state analyses. In addition, the following investigations are discussed: the formation of mullite precursors by the adsorption of monosilicic acid to Al(OH)3, the spontaneous reduction of Au(III) to Au(0) by adsorption of Au(III) to Al(OH)3, the mechanism of concentration of Co2+, Tl+, Pb2+, Pt2+, Au+, and Pd2+ in marine ferromanganese crusts, the chemical state of Fe3+ adsorbed to a cation-exchange resin, and the adsorption mechanism of PO43- in this Fe3-type cation exchange resin, which can be used as an adsorbent for undesired anion species such as arsenic, selenium, and phosphorous.
